1. Field of the Invention
The present invention relates to a gas supplying system and a gas supplying apparatus and, more particularly, to a gas supplying system and a gas supplying apparatus for use with an industrial manufacturing apparatus such as a semiconductor manufacturing apparatus.
2. Description of Related Art
In a semiconductor manufacturing process, a supply gas such as a corrosive gas, toxic gas or combustible gas is conventionally used for etching or the like of photoresist working.
In general, such photoresist working (photoresist coating, exposure, development, and etching) is repeated plural times in the semiconductor manufacturing process, and so, a gas supplying apparatus for supplying a corrosive gas or the like as required is practically used in the semiconductor manufacturing process.
In most cases, a plurality of kinds of supply gases or a supply gas of the same kind having different concentrations are used for the photoresist working.
A desired kind of supply gas is prepared by mixing a plurality of kinds of corrosive gases or component gases in a closed chamber. Further, a desired concentration of supply gas is prepared by mixing an inert gas with a corrosive gas or the like.
Further, it is desired to accurately control an amount of supply gas to be used for etching or the like in response to recent high integration and precision of semiconductors.
Also, the demands regarding a supply timing and a supply speed of the supply gas have become severe in response to the need of shortening of a manufacturing process time.
In a conventional gas supplying apparatus for supplying a corrosive gas or the like, after the corrosive gas is supplied to an etching device or the like, it remains in a gas supply pipe, and is allowed to stand until a fresh corrosive gas is supplied in the next cycle. As a result, an inner wall surface of the gas supply pipe formed of metal is corroded by the residual corrosive gas, and impurities or particles due to the corrosion are mixed into the corrosive gas to be supplied in the next cycle, thus adversely effecting the semiconductors.
Further, in the case of using a chamber or the like for mixing a plurality of kinds of corrosive gases to form a supply gas therein, a residual supply gas even in a small amount remaining in the chamber causes a change in component of a supply gas to be supplied in the next cycle. This may arise a greatly adverse effect on the photoresist working as depending upon the kinds of the residual supply gas and the supply gas to be next supplied, causing a deterioration in quality of semiconductor products.
In the case where the residual supply gas is a combustible gas, there is a possibility that the combustible gas even in a small amount will react with a supply gas to be next supplied to occur combustion or explosion by the combustible gas as depending upon the kind thereof.
Even if the explosion does not occur, a reaction product becomes an impurity as particles to adversely effect the semiconductor manufacturing process.
As measures against the above problems, after mixing desired amounts of plural kinds of corrosive gases together in the chamber to form a desired supply gas and then supplying the desired supply gas to the semiconductor manufacturing process, a residual supply gas remaining in the chamber is substituted by an inert gas such as a nitrogen gas.
The substitution of the residual supply gas by the inert gas is performed by the following known methods.
(1) A method of sucking the residual supply gas remaining in the chamber by using a vacuum pump.
(2) A method of forcibly introducing the inept gas into the chamber to purge the residual supply gas remaining in the chamber.
However, according to the method (1), as the vacuum pump is separate in position from the chamber, the chamber cannot be efficiently evacuated by the vacuum pump. Furthermore, in the case of a corrosive gas, it strongly sticks to the inner wall surfaces of the chamber and the gas supply pipe formed of metal, so that the corrosive gas cannot be perfectly removed.
In general, it is unsuitable to locate the vacuum pump in the vicinity of the gas supplying apparatus from the viewpoints of maintenance and space, and the vacuum pump is therefore located apart from the gas supplying apparatus. As a result, it is necessary to provide a long suction pipe for connecting the chamber to the vacuum pump. Since the suction pipe is long, a suction resistance becomes large to hinder the evacuation of the chamber, so that the residual supply gas remaining in the chamber cannot be perfectly removed.
Furthermore, in the case of a corrosive gas, since the residual corrosive gas to be sucked through the suction pipe by the vacuum pump has a high concentration, the inner wall surface of the suction pipe is corroded by the corrosive gas flowing in the suction pipe.
On the other hand, according to the method (2), much time is required to dilute the residual supply gas. For example, when a nitrogen gas is forced into the chamber under the pressure of 2 kg/cm.sup.2, about one hour is required to dilute the residual supply gas down to a concentration of 0.05 ppm which is a reference value less adversely effecting a supply gas to be next supplied. Accordingly, a required period of time for the semiconductor manufacturing process is elongated to reduce a production efficiency.
Incidentally, it is desired to accurately supply a small amount of corrosive gas or the like with an error of 1% or less in terms of a mass flow. Thus, the requirement for a high accuracy has become increasingly severe. To meet this requirement, a flow control valve having a high accuracy and a high responsiveness is used.
In such a flow control valve for accurately controlling a supply amount of a corrosive gas or the like is provided with a mass flow sensor capable of measuring a mass flow with a high accuracy and a high responsiveness. That is, a pair of self-heating temperature sensing elements each having a large temperature coefficient are wound around a thin conduit tube at its upstream and downstream ends to form a pair of heat sensitive coils. A bridge circuit is formed of each heat sensitive coil, and a temperature of each heat sensitive coil is controlled to a constant value. A mass flow of the corrosive gas flowing in the conduit tube is computed from a potential difference between both the bridge circuits.
The conduit tube is formed as a tube having an inner diameter of 0.5 mm and a length of 20 mm, which is made of SUS 316, for example. The small inner diameter is intended to measure a small amount of flowing gas.
Each heat sensitive coil is formed by winding 70 turns of heat sensitive resistance wire having a diameter of 25 .mu.m around the conduit tube. The heat sensitive resistance wire is formed of a material having a large temperature coefficient, such as iron or nickel alloy. Each heat sensitive coil is bonded to the conduit tube by UV curing resin or the like to form a sensing element.
While such a flow control valve is widely used in a corrosive gas supplying apparatus for the semiconductor manufacturing process, it is difficult to perfectly remove the corrosive gas remaining in the thin conduit tube provided in the flow control valve. Further, when an inner wall surface of the conduit tube is corroded by the corrosive gas, the accuracy of the mass flow sensor is reduced. Accordingly, the corrosive gas cannot be supplied with a high accuracy to cause a remarkable decrease in yield in the semiconductor manufacturing process.